Shallow water platform

ABSTRACT

A device is described for hull in all essentials symmetrical about a centre axis and arranged to be accommodated resting on a seabed, where the hull is provided with several support means which in an active position protrudes downwards from a hull bottom and is arranged to maintain the centre axis of the hull in a specified position by each support mean being arranged for vertical displacement in relation to one or both of the hull and the seabed. It is also described a method of positioning a hull in all essentials symmetrical about a centre axis on a seabed.

The invention concerns a hull that is generally symmetrical about acentre axis and arranged to be placed stationary and resting on aseabed. The invention also concerns a method of positioning such a hullon a seabed.

Platforms that are used e.g. for exploration, production and storage ofoil and gas in/from submarine structures and are floating both duringtransportation and operation, may also be used when resting on theseabed in shallow waters. In particular, platforms having a circular, orsubstantially circular, horizontal cross-section (e.g. of the type knownfrom the applicant's own patent publication WO02090177), where thebuoyancy volume is rather large and the platform therefore can besubmerged a relatively great amount without having too small afreeboard, are particularly suitable.

The disadvantage of having platforms of this type rest on the seabed isthat the platform might assume an inclined position as its orientationis depending on the seabed conditions. If this is to be avoided, theseabed must be prepared, for example by filling and levelling, anoperation which is time consuming and costly.

So-called jackup platforms, which in an operative position are restingon vertically displaceable legs and where the entire platform hull isabove sea level, are not dependent of such demanding and costlypreparation of the surface they will be placed upon, by that very factthat the position of the platform can be adjusted with individualregulation of each leg by means of built-in jackup mechanisms. Thedisadvantage with this type of platform is its low load-carryingcapacity during transportation in a floating condition, and the shape ofthe hull and the positioning of the legs makes is vulnerable for ice,such that it is not suited for use for example in arctic regions, withits combination of demanding ice conditions, shallow waters and roughseas during bad weather.

The object of the invention is to mitigate or to reduce at least one ofthe disadvantages with the prior art, or at least provide a usefulalternative to the prior art.

The object is achieved by features indicated in the description belowand in the following patent claims.

It is provided a floatable hull which is symmetrical or essentiallysymmetrical about a vertical centre axis, for example cylindrical orpolygonal, and is provided with a plurality of support means that atleast in an operative position protrudes downwards from a hull bottomand that during lowering of the hull towards a seabed is arranged toprovide an adjustable support of the hull relative to the seabed.

The support is provided by the support means resting against the seabedor penetrating into uncompacted material below the seabed, as thesupport means are pressed and/or sucked down into the uncompactedmaterial until sufficient friction force between the uncompactedmaterial and the support means is provided for stable support of thehull.

In the following, the terms “suction foot/feet” are used for the supportmeans of the type that in principle works as in a per se known suctionanchor, as the support means are formed as a cup with a downward facingopen end, and where the cup is connected to an evacuation system thatcan provide an underpressure inside the cup such that it is sucked downinto the seabed on which it is resting. Both water and uncompactedmaterial can in principle be evacuated from the vessel, in order for thesuction foot to be brought sufficiently down into the uncompactedmaterial. It is obvious that the penetration is also affected by themass of the hull and the buoyancy of the hull, i.e. the net downwardpressure of the hull.

The expression “support leg” refers to a support means configured to beresting on a supporting surface, for example the seabed or on preparedfoundation materials, without penetrating much into the surface.Typically, a lower part of the support means in this shape will have anexpanded, downward facing end surface.

The support means can be formed as vertical columns, or similar. Theadjustment can be provided by the support means being formed as suctionfeet which by differentiated internal underpressure in the suction feetare pressed into the uncompacted material below the seabed.Alternatively, the support means can be formed as support legs restingon the seabed. When the support means are formed as support legsarranged for contact with the seabed, the support means are preferablyarranged to vertical displacement relative to the bottom of the hull.Vertical displacement can also be applied when the support means areformed as suction feet. Each vertically displaceable support meanscomprises means for individual vertical displacement. Thereby, thesupport means can be adjusted individually, and the hull can thereby besupportedly arranged against the seabed with the vertical centre axisindependent of the slope and evenness of the seabed.

In a first aspect, the invention relates more specifically to a hulldevice, essentially symmetrical about a centre axis and arranged suchthat a hull side exhibits a predetermined freeboard, characterized inthat the hull is provided with several support means which in an activeposition project downwards from a hull bottom and are arranged tomaintain the centre axis of the hull in a specified position by eachsupport means being arranged for vertical displacement in relation toone or both of the hull and the seabed.

The support means can be provided with suction feet arranged to bepressed into uncompacted material below the seabed, and which is rigidlyconnected to the hull bottom.

The support means can be provided with suction feet configured to bepressed into uncompacted material below the seabed, or may be providedwith support legs arranged to about against the seabed, and which areindividually vertically displaceable in relation to the hull bottom.

The support means may in a retracted position be at least partlyaccommodated by respective pockets inside the hull. The outer shape ofthe hull is thereby maintained, as the hull and the support means showless vulnerability against harmful impacts of ice.

The vertical displacement of each support means is provided by one ormore actuators associated with the hull and the support means.

The hull may comprise a ballasting system arranged to be able to lowerthe hull until a hull side exhibits a specified freeboard.

In a second aspect the invention relates to more specifically a methodfor positioning a hull, that is essentially symmetrical about a centreaxis, on a seabed, characterized in that the method comprises the stepsof:

-   -   a) arranging the hull floating on a specified location above the        seabed; ballasting the hull such that a hull side exhibits a        predetermined freeboard; and    -   b) displacing several support means vertically downwards to        contact with the seabed, possibly into uncompacted material        below the seabed to obtain stable support of the hull.

The method may comprise the further step of:

-   -   d1) displacing the support means provided with suction feet in        relation to the seabed during ballasting of the hull by applying        a differentiated underpressure to the suction feet.

Alternatively, the method may comprise the further step of:

-   -   d2) displacing the support means provided with support legs in        relation to the hull bottom by means of one or more actuators.

The method may comprise the further step of:

-   -   e) locking each of the support means relative the hull by means        of the respective actuator.

In the following, an example of a preferred embodiment is described,illustrated in the attached drawings, where:

FIG. 1 shows in perspective, at an angle seen from below, a principledrawing of a hull according to a first embodiment of the invention, asthe hull is provided with fixed support means in the form of suctionfeet;

FIG. 2 shows in perspective, at an angle seen from below, a principledrawing of a hull according to a first embodiment of the invention, asthe hull is provided with vertically displaceable support means in theform of suction feet;

FIG. 3 shows in a side view a principal sketch of a hull according toprior art arranged on a prepared surface on a sloping seabed;

FIG. 4 shows in a side view a principal sketch of the hull according toa first embodiment of the invention supported on a sloping seabed bymeans of supporting means in the form of fixed suction feet pressedpartly into a uncompacted material below the seabed; and

FIG. 5 shows in a side view a principal sketch of the hull according toa first embodiment of the invention supported on a sloping seabed bymeans of supporting means in the form of vertically displaceable supportfeet resting against the seabed.

In the figures, the reference number 1 indicates a cylindrical hull witha vertical centre axis 11 in its operative position and partly with aninclined outwardly sloping upper portion 1 a. Even though the hull 1here is shown symmetrically, the invention also covers other hull shapeswhich are essentially symmetrical about a vertical centre axis, forexample a polygonal hull. Due to stability considerations during thefloating condition of the hull 1 the hull 1 is provided with a hullskirt 13 of a per se known type.

In a first embodiment, illustrated in FIGS. 1 and 4, the hull 1 isprovided with several fixedly arranged support means 17 protrudingdownwards from a hull bottom 12. In this embodiment the support means 17are provided with suction feet 17′ which by means of a vacuum plant (notshown) a differentiated underpressure may be applied such that thesuction feet 17′ by means of the internal underpressure are sucked downinto the uncompacted material 31 below a seabed 3. By controlling theunderpressure individually for each suction foot 17′, the hull 1 can belevelled even if the seabed 3 is sloping.

In a second embodiment, illustrated in FIGS. 2 and 5, the hull 1 isprovided with several pockets 15, each accommodating a support means 17provided with support legs 17″ arranged to rest against the seabed 3 andwhich is vertically displaceable between an inactive positionessentially retracted into the hull 1 and an active position partlyprotruding downwards from the hull bottom 12 through respective openings121 in the hull bottom 12. The vertical displacement of each of thesupport means 17 is provided by means of one or more actuators 16, shownhere schematically as one actuator, as the actuators form a connectionbetween the hull 1 and the support means 17.

The support means 17 with suction feet 17′ according to the firstembodiment may be combined with the vertical displacement of the supportmeans 17 according to the second embodiment. This combination will beable to provide better certainty that the hull 1 being capable of beinglevelled also when the condition of the uncompacted material obstructsor restrains the penetration of the suction feet 17′.

Even though the embodiment according to FIGS. 1 and 4 and according toFIGS. 2 and 5 is shown with three support means 17, the effect of theinvention can in principle be achieved by means of only two supportmeans 17, as a portion of the periphery of the hull bottom 12 canconstitute a support means abutment portion which rests against thehighest area of the portion of the seabed 3 on which the hull 1 is to besupported, while the adjustment of the inclination of the hull 1 isprovided by the two support means 17 by suction or vertical displacementor a combination thereof.

The hull 1 may be adapted to the condition of the seabed 3 by providingthe hull with support means 17 of the type most suitable for theexisting conditions. This is especially obvious in the embodiment wherethe support means 17 are vertically displaceable.

The hull 1 is in a per se known way provided with a ballasting system 14for trimming the hull 1, including giving the hull 1 sufficientstability in a floating condition during transportation betweenlocations, and to submerge the hull 1 until a hull side 18 exhibits aspecified freeboard 181 in an operative condition where the hull 1 is tobe supported by the seabed 3.

Devices (not shown) for operation of the actuators 16 and pumps etc.(not shown) for providing a suction effect when the suction feet 17′ areplaced or released, is not shown. These can be arranged in the hull 1,but they can also be arranged on tender vessels (not shown) if forexample there is a lack of space in the hull 1.

1. A hull substantially symmetrical about a centre axis and configuredto be arranged such that a hull side exhibits a predetermined freeboard,the hull comprising: several support members that in an active positionprotrude downwards from a hull bottom and are arranged to maintain thecentre axis of the hull in a specified position, each support memberbeing arranged for vertical displacement in relation to one or both ofthe hull and a seabed.
 2. The device of claim 1, wherein the supportmembers are provided with suction feet arranged to be pressed intouncompacted material below the seabed, and which is rigidly connected tothe hull bottom.
 3. The device of claim 1, wherein the support membersare provided with suction feet arranged to be pressed into uncompactedmaterial below the seabed, or provided with support legs arranged tocontact the seabed, and which are individually vertically displaceablein relation to the hull bottom.
 4. The device of claim 3, wherein thesupport members in a pulled up position are at least partly accommodatedby respective pockets inside the hull.
 5. The device of claim 1, whereinthe vertical displacement of each support member is provided by one ormore actuators connected to the hull and the support members.
 6. Thedevice of claim 1, wherein the hull comprises a ballasting systemarranged to be able to submerge the hull until a hull side exhibits apredetermined freeboard.
 7. A method of positioning a hull, that isessentially symmetrical about a centre axis, on a seabed, comprising:arranging the hull floating on a specified location above the seabed;ballasting the hull such that a hull side exhibits a predeterminedfreeboard; and displacing several support members vertically downwardsand into abutment with the seabed, potentially into uncompacted materialbelow the seabed to stable support of the hull.
 8. The method of claim7, wherein the method further comprises: displacing the support membersprovided with suction feet in relation to the seabed during ballastingof the hull by applying the suction feet a differentiated underpressure.9. The method of claim 7, wherein the method further comprises:displacing the support members provided with suction feet in relation tothe hull bottom by means of one or more actuators.
 10. The method ofclaim 8, wherein the method further comprises: locking each of thesupport members relative the hull by means of the respective actuator.